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Corralling Cercospora | When Cercospora Hits, Strobilurin Fungicides Should Help Control It

Macon LaFoe knows it’s not if his soybeans and corn will face big pressure from cercospora,but the proverbial when. However, fungicide treatments can KO the disease and could also produce higher yields.

Cercospora is nasty fungus that comes in several variations, which can prevent beans from maturing properly and cause gray leaf spot in corn. Left uncontrolled, the disease can ruin leaves, thwart ears, lead to lodging of weak stalks on corn and decrease soybean yields 25% or more.

Timely fungicide treatments will help prevent or control the disease. LaFoe has had success in using fungicides in both corn and beans on his Oak Ridge, LA, farm. That’s been through periodic, as-needed sprays.

Even though University of Illinois and University of Tennessee research indicates the fungus that causes cercospora, frogeye leaf spot and other diseases has shown resistance to strobilurin fungicides in a Tennessee field, LaFoe will continue treatments when they’re needed. The resistance has not been documented beyond Tennessee.

LaFoe’s corn and bean acres are on former cotton ground that was mostly disease-free. Not enough corn has been produced to generate severe disease problems.

“We nearly always spray soybeans with a strobilurin fungicide because of the potential for soybean rust and other diseases,” he says. “We typically don’t have as much disease pressure on corn. But the more corn we grow, the more disease we’re likely to see.”

His rotation is about two-thirds corn and one-third soybeans. Corn yields are 190-220 bu. under irrigation. Beans push 60 bu. There’s also some wheat on the 2,500-acre farm. All crops faced pressure in 2009 due to an extra dry spring and early summer that turned to extra wet weather in late summer and early fall.

“We didn’t spray much corn because it was so dry and we didn’t foresee much disease pressure,” LaFoe says. “That wasn’t an ordinary year. We usually see a wetter, cooler spring. And that often promotes disease problems.”

In those years, he applies fungicides to beans and corn and doesn’t mind the $14-15/acre cost it involves.

“We usually use one or two programs,” he says. “We either apply 10 oz. of Stratego or 14 oz. of Quilt. They provide good control for us on cercospora infestations.”

“We’ve found that the quality of our beans has improved with fungicide treatments,” LaFoe points out. “From what we’ve seen in corn test plots, there’s a chance we can improve our corn yields, as well.”

The corn test plots, conducted in conjunction with Bayer CropScience, involved a check plot, a plot treated with 10 oz. of Stratego and a plot with a 12-oz. Stratego application.

“With the 10-oz. rate, we saw a 1-bu./acre higher yield than with the check,” says LaFoe. “But with the 12-oz. treatment we saw a 10-12-bu. improvement in yields.

Impact on other diseases

Larry Dunkle, USDA-ARS plant pathologist at Purdue University, points out that gray leaf spot is normally considered “a serious disease” in the Corn Belt, adding that minimum- and no-till production has likely added to the amount of disease pressure corn and beans can face.

“Tillage operations that reduce the amount of surface residue from the previous crop should decrease disease severity,” says Dunkle. “But spores of the cercospora fungus can be carried long distances by wind, so the inoculum doesn’t necessarily come only from the previous year’s crop.”

Carl Bradley, University of Illinois plant pathologist, adds that cercospora can cause several diseases in soybeans, including frogeye leaf spot and cercospora blight. “Fungicide products that contain a strobilurin are generally effective for control of these diseases,” he says. “In research trials we've conducted, strobilurin fungicides have reduced frogeye leaf spot severity.”

He notes that when comparing yields of treated corn or beans with a check plot or plots with a different rate of fungicide, growers should have more than one strip for each treatment so that trials are replicated. “It’s important to look at replicated trials to make sound observations,” he says. “There can be a lot of variability in a field.

“In my studies, if you have a lot of disease pressure, you can get good control of gray leaf spot with products that contain a strobilurin fungicide. We only see the 10-20-bu. differences in yields when there is pretty good disease pressure out there. For the most part, the disease has to be there before we see the big consistent yield increase (from fungicide treatments),” says Bradley.

Control measures

When facing the threat of cercospora or other disease pressure, growers should look for corn hybrids or bean varieties with disease resistance or tolerance. “Some soybean varieties have good resistance to frogeye leaf spot,” he says. “Varieties with the RCS3 gene hold up effectively.

“In corn, there are no hybrids with complete resistance, but there are some with some tolerance. Growers should also scout and monitor their fields,” he says. “If you have conditions that promote disease (wet weather, high relative humidity, etc.), you need to look at spraying a fungicide.

“Continuous corn is always a risk because cercospera overwinters in the residue in those fields,” Bradley points out.

Of course, the price of corn or soybeans may dictate whether the extra $15+/acre cost is warranted, he says, noting that “corn at $4.50-5 (or higher) would probably be sprayed more openly than corn at under $3.”

LaFoe doesn’t mind the cost if he can boost yields. “I think it’s a good investment to put down a fungicide on corn and beans, especially if we see the wet spring weather,” he says.

Meanwhile, the Fungicide Resistance Action Committee (FRAC), an international committee that evaluates fungicides' likelihood of developing resistance, has determined that strobilurin fungicides have been deemed high risk for fungal pathogens developing resistance to them. "Plant pathogenic fungi developing resistance to strobilurin fungicides is not new," says Bradley. "This has already occurred in potatoes and other crop and disease systems where multiple fungicide applications occur during the growing season."